Project Summary Nitric oxide (NO), a ubiquitous signaling molecule, is important for most physiological processes including bone homeostasis. However, extensive in vitro and in vivo studies that have assessed the role of NO in bone biology have often yielded contrasting results. This is at least in part due to the fact that pharmacologic inhibition of nitric oxide synthases (NOS) or genetic models of NOS deficiency are limited by the redundancies of the NOS isoforms and cannot address the cell- autonomous roles of NO. Argininosuccinate lyase (ASL), is a urea cycle enzyme is not only required for the de novo synthesis of arginine, the substrate for NOS, but also to maintain the structural integrity of a NO-synthesis complex containing NOS, argininosuccinate synthase (ASS1), the arginine transporter CAT-1, and HSP90. Loss of ASL leads to non-redundant and cell-autonomous loss of NOS-dependent NO production and thus ASL deficiency (ASLD) is a human genetic disorder of NO production. The overall goals of this proposal are to study the role of NO in bone turnover, density, and architecture in a human model of NO deficiency and to understand the mechanistic basis by which NO affects bone metabolism. By leveraging an ongoing trial in this rare genetic disorder, we will address these specific questions: 1) Do patients with ASLD have abnormalities in bone turnover and bone mass and does NOS-independent NO supplementation affect these endpoints? 2) Do patient- derived induced pluripotent stem cells (iPSC) show differentiation defects along the osteoblastic lineage and how does this impact osteoclastic differentiation? 3) Do osteoblasts derived from patient- iPSC exhibit dysregulation of NO production due to dominance of a caveolin-dependent negative regulatory NOS complex? These studies could have a significant impact on the basic understanding of bone biology and foster translational studies in utilizing NOS-independent NO supplementation as a therapeutic intervention in the more common disorders like osteoporosis.